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Failure of Automotive Steels Resistance Spot Welds under Mode I

Nadimi, Nima | 2021

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 54700 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Pouranvari, Majid
  7. Abstract:
  8. Automotive steels are dominant material for the manufacturing of automotive structures and components. Since an automotive body is mostly assembled by spot welding, spot weld failure in different loading conditions has a great influence on the crashworthiness of vehicle. Therefore, investigation of microstructure and failure behavior of resistance spot welded automotive steels is an important issue. The first part of the research is dedicated to microstructural evolution and fusion zone hardness of spot welded automotive steels based on optical and SEM micrographs and hardness measurements. In the FZ of automotive steels, except for austenitic steels, a mainly martensitic microstructure was observed due to high cooling rate of the RSW process. According to the results of current and previous researches, a regression model was developed to predict the FZ hardness of automotive steels. In the second part of the study, effect of welding current and nugget diameter of joints on the mechanical properties and failure mode in cross tension test was examined. During the cross tension test the notch at the sheet/sheet interface experiences mode I loading condition. As the welding current increased, the nugget size became larger and the failure mode changed from IF or PIF to plug failure mode. However, in cross tension test the welds are very prone to plug failure mode, even with a small button diameter. Third part of the study aims at understanding interfacial and plug fracture mechanisms for developing a model to predict cross tension strength for both IF and PF mode. It was found that for most of the high strength steels in particular AHSS, interfacial failure is controlled by mode I stress intensity factor at the notch tip. Nonetheless, interfacial failure of LCS, 304 and DP450 spot welds in cross tension test is controlled by FZ hardness parameter. For plug failure mode, two main fracture mechanisms are identified: strain localization and ductile shear fracture
  9. Keywords:
  10. Automotive Steels ; Resistance Spot Welding ; Fracture Mechanism ; Cross Tension Test ; Microstructure ; Mode I Crack

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